The invention relates to a system for quickly reversing the direction of rotation of a single phase AC electric motor.
When trying to quickly reverse a single phase AC motor, the mere reversal of voltage polarity applied to the main and auxiliary windings of the motor while it is running will not reverse the motor. That is, the applied voltage to the main winding reverse polarity each half cycle, regardless of whether it is positive during the first or second half cycle, and without interaction with the auxiliary winding, no reversal can take place.
One possibility for reversing the motor is to stop the motor, and then reenergize the start winding in the opposite polarity relative to the main winding, or vice versa. This would make the motor rotate in the opposite direction. This method of stopping the motor before reversal is undesirable or unacceptable in various applications where instant reversal while running in a certain direction is needed or desired, for example an electric hoist motor.
In another known reversing system, a mechanical centrifugal disconnect switch is used in combination with a mechanical reversing switch manually operated by the user. The user operated reversing switch controls a first set of external switches for applying a first polarity voltage to the start winding from the AC source, and controls a second set of external switches for applying a second opposite polarity voltage to the start winding from the AC source. The centrifugal switch is normally closed at initial energization of the motor, to connect the start winding to the AC power source. The centrifugal switch automatically opens when the motor speed is approximately 80% of synchronous speed, causing the start winding to be disconnected from the AC power source. The centrifugal switch has two pairs of contacts, a first pair for supplying current through the start winding in one direction when its associated set of external switches are closed, and a second pair of contacts for supplying current through the start winding in the opposite direction when its associated set of external switches are closed. At initial energization of the motor, both pairs of contacts are closed and one set of external switches is closed, to supply current in one direction through the start winding. As the motor starts to rotate, a wiper actuator is frictionally dragged by motor rotation to a first given position adjacent the first pair of contacts, and when the centrifugal actuator operates, it causes the wiper actuator to open the first pair of centrifugal switch contacts to disconnect the start winding from the AC power source. During the running mode, when the operator manually actuates the external reversing switches, a different electrical circuit is completed, which is through the closed second pair of centrifugal switch contacts, to supply current through the start winding in the opposite direction and hence apply reverse torque to the motor. As the motor slows down, the centrifugal switch closes which in turn moves the wiper actuator axially and closes the first pair of centrifugal switch contacts which are now in an open circuit. When the motor starts rotating in the opposite direction, the wiper actuator is frictionally dragged to a second position adjacent the second pair of centrifugal switch contacts. When the motor speed reaches approximately 80% of synchronous speed in such opposite direction of rotation, the centrifugal switch opens which in turn moves the wiper actuator axially to open the second pair of contacts and disconnect the start winding from the AC power source. The cycle may repeat, with the operator manually actuating the external reversing switches to complete an electrical circuit through the first pair of contacts. This type of switching arrangement is typically known in the trade as an "iron fireman" arrangement. Another example is a General Electric Reverswitch R98-1, 8422. Such arrangement provides instant reversing (i.e., immediate application of reverse torque) because the wiper actuator has been moved by rotation of the motor in its running condition to be ready and in the proper position for motor reversal. While this type of arrangement has proven useful for its intended purpose, it is nonetheless subject to the problems inherent in a mechanical type actuation system, including limited life, excessive contact arching due to random switching, fatigue, friction, particularly the dragging of the wiper actuator, vibration, mounting position, contact and wiper actuator wear, and so on.
The present invention addresses and solves the above noted single phase motor instant reversing and other problems in a particularly simple and effective electronic system electrically sensing polarity reversal and automatically reconnecting the start winding to the AC source. A particularly desirable feature of the invention is that the instant reversing technique is independent of the single phase motor speed at which the reversing switch is operated by the user. For example, the operator of a hoist can go instantly from up to down without having to momentarily delay the reversing switch in a central off position before continuing to the reversed direction position. In the present invention, polarity reversal is instantly detected and recognized by sensing when one of the main and auxiliary winding voltages shifts from a leading to a lagging position relative to the other, and the start winding is instantly reconnected to the AC source such that the reversed polarity between the main and start windings begins applying reverse torque to the motor, slowing the motor down and accelerating it in the opposite direction of rotation.
Another desirable feature of the invention is that an initiation driver pulse which triggers connection of the start winding to the AC source is reapplied during each cycle of the AC source until the motor speed decreases to zero, to thus repeatedly insure application of reverse torque. This is particularly desirable for high inertia loads where it may not be possible to quickly reverse the single phase AC motor or reduce its speed to zero.